Optical add-drop multiplexers (OADM) and optical hubs (or wavelength cross-connect) enable dropping, adding, or switching optical signals on an individual wavelength basis, without the need for converting them into electric signals. Recent wavelength-division multiplexed (WDM) optical networks use those all-optical network devices to provide interconnections in ring, mesh, and other complicated topologies. There is, accordingly, a growing need for techniques to optimize an increasingly complex network design.
In the process of designing a WDM optical network, a network design apparatus is used to, for example, create a layout of communication devices and set up a route of demand traffic. During this course, simulation tools are used to determine whether all WDM channels can deliver signals, while the network may not necessarily need all of them.
FIG. 13 illustrates determination of whether every wavelength channel can deliver signals. For example, a network design apparatus is used to design an optical network that conveys main signal traffic at the rate of 10 Gbps. FIG. 13 illustrates a full set of WDM main signals with wavelengths of λ1 to λn. The network design apparatus tests each of those signals to determine whether they can all deliver signals. While not all those wavelength channels are necessary to implement 10-Gbps signal transmission, the network design apparatus determines that it is not possible to realize the desired network if even a single wavelength channel is found incapable of delivering a signal properly.
Suppose now that the above 10-Gbps optical network is to be expanded to a 40-Gbps network. This means that new wavelength paths will be added to the existing 10-Gbps network. The network design apparatus determines whether all wavelength channels can deliver signals, similarly to the foregoing case of 10 Gbps. This determination for the purpose of 40-Gbps expansion is performed on the same full set of wavelength channels as in the case of the 10-Gbps optical network.
As described above, the network design apparatus has once determined in the 10-Gbps network design that all wavelength channels can deliver signals. This may not necessarily mean that the same set of wavelength channels can work similarly in the 40-Gbps design; some wavelength channels may be found incapable of delivering signals properly.
When it is determined that all wavelength channels can be used for the 40-Gbps design, the network design apparatus selects wavelength channels that are not used in the existing 10-Gbps optical network and assigns them to the network as its additional channels. For example, the network design apparatus selects optical paths from the unused wavelength channels in FIG. 13 and adds them to the existing optical network. This operation makes it possible to expand an optical network from, for example, 10 Gbps to 40 Gbps. When, on the other hand, there is even a single wavelength channel that cannot deliver signals, the network design apparatus finds it impossible to expand the existing optical network to 40 Gbps, in spite of the presence of usable wavelength channels.
For example, Japanese Laid-open Patent Publication No. 2004-048477 proposes a network design apparatus which determines whether signals can be delivered or not, based on normalized noise levels. The proposed apparatus normalizes beforehand the amount of noise produced in each node, as well as the tolerance thereto, according to segment type, segment loss, and the number of wavelengths.
As can be seen from the above discussion, the conventional network design method is unable to provide a solution to expand an existing optical network in the case where an incapable wavelength channel is included in the full set of wavelength channels, even though other wavelength channels can deliver signals.